1. Technical Field
The present disclosure relates to a method of packaging a MEMS transducer device and to a packaged MEMS transducer device. The MEMS transducer device is, in particular, a membrane-based speaker.
2. Description of the Related Art
FIG. 1 shows, in top plan view, a MEMS speaker 1 comprising a plurality of membranes 2, which are arranged so as to form a planar array 3. For example, the MEMS speaker 1 is formed by 1024 membranes, arranged in thirty-two rows and thirty-two columns. Each membrane 2 forms a speaker unit 10 (also known as “pixel”), which can be driven selectively by respective conductive pads 5.
FIG. 2 shows, according to a cross-sectional view taken along the line of section II-II of FIG. 1, a membrane-based MEMS transducer, for example a MEMS speaker 1. For reasons of simplicity of representation, FIG. 2 shows a semiconductor body 9 including a plurality of membranes 2, but the means for actuation of such membranes 2 for generating, in use, sound waves are not illustrated. Known to the art are MEMS speakers provided with membranes 2 actuated by means of electrodes appropriately biased (electrostatic actuation), or else MEMS speakers provided with piezoelectric actuators, which are set fixed with respect to the membranes 2 and can be actuated so as to generate controlled deflections of the membranes 2 themselves. Irrespective of the actuation method, the controlled movement of the membranes 2 (in the direction Z) generates sound waves, according to the known operation of a speaker. Each membrane 2 of the MEMS speaker 1 is suspended over a respective cavity 14. Each cavity 14 is separated, from cavities 14 adjacent thereto, by walls 12 of the semiconductor body 9. In use, each cavity 14 has the function of acoustic cavity (or resonant cavity) of the MEMS speaker 1. It is evident that the dimensions of each cavity 14 are designed to optimize the characteristics of the sound generated by the MEMS speaker 1 during use.
At the end of the steps of machining of the semiconductor body 9 (in particular, formation of the membranes 2 and of the cavities 14), the MEMS structure thus obtained is housed in a package 16 so as to support it and protect it.
It is expedient for the steps of coupling of the semiconductor body 9 with the package 16 not to modify the internal volume of the cavities 14 (in particular, reducing it). Use of epoxy-based, or polyamide-based, or acrylic-based glues or resins, or other glues typically used for bonding semiconductor substrates or wafers, does not meet the aforementioned preferences. Once again with reference to FIG. 2, represented schematically therein is the effect of reduction of the internal volume of the cavities 14 on account of the use of resins of a known type. The resin or glue used extends as far as the interface between the portion of package 16 to which the semiconductor body 9 is coupled (interface regions 18′) and also laterally to such interface 18′, partially rising within the cavities 14 (lateral regions 18″) and partially filling the cavities 14.
A possible solution to this problem is the use, as an alternative to resins or glues, of biadhesive tapes, for example die-attach films (DAFs). However, the biadhesive tapes currently present on the market are characterized by a Young's modulus (or elastic modulus) of a relatively high value (typically between approximately 200 and 800 MPa, according to the type of tape used), and cause warpage of the MEMS speaker 1, and in particular of the membranes 2, which is not easy to control. A deformation of the membranes 2 seriously jeopardizes the acoustic characteristics of the MEMS speaker 1 and is consequently a markedly undesirable effect.